linux-kernel - Re: sched: tweak select_idle_sibling to look for idle threads

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Date:	Sat, 7 May 2016 09:24:17 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Mike Galbraith <mgalbraith@...e.de>
Cc:	Peter Zijlstra <peterz@...radead.org>, Chris Mason <clm@...com>,
	Ingo Molnar <mingo@...nel.org>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: sched: tweak select_idle_sibling to look for idle threads

On Sun, May 01, 2016 at 11:20:25AM +0200, Mike Galbraith wrote:
> On Sun, 2016-05-01 at 10:53 +0200, Peter Zijlstra wrote:
> > On Sun, May 01, 2016 at 09:12:33AM +0200, Mike Galbraith wrote:
> > > On Sat, 2016-04-30 at 14:47 +0200, Peter Zijlstra wrote:
> > 
> > > > Can you guys have a play with this; I think one and two node tbench are
> > > > good, but I seem to be getting significant run to run variance on that,
> > > > so maybe I'm not doing it right.
> > > 
> > > Nah, tbench is just variance prone.  It got dinged up at clients=cores
> > > on my desktop box, on 4 sockets the high end got seriously dinged up.
> > 
> > Ouch, yeah, big hurt. Lets try that again... :-)
> 
> Yeah, box could use a little bandaid and a hug :)
> 
> Playing with Chris' benchmark, seems the biggest problem is that we
> don't buddy up waker of many and it's wakees in a node.. ie the wake
> wide thing isn't necessarily our friend when there are multiple wakers
> of many.  If I run an instance per node with one mother of all work in
> autobench mode, it works exactly as you'd expect, game over is when
> wakees = socket size. It never get's near that point if I let things
> wander, it beats itself up well before we get there.

Maybe give the criteria a bit margin, not just wakees tend to equal llc_size,
but the numbers are so wild to easily break the fragile condition, like:

if (master * 100 < slave * factor * 110)
        return 0;

And since you accumulate wakee number (and decay at HZ), this check tends to
not satisfy ever?

if (slave < factor)
	return 0;